This invention relates to the compensation of chromatic dispersion, hereinafter referred to as dispersion, in optical transmission systems. In particular, although not exclusively, this invention relates to the compensation of dispersion in Wavelength Division Multiplexed (WDM) optical communications systems.
Generally, chromatic dispersion is the dependence of wave velocity on wavelength as a wave travels through a medium. In the field of optical communications, chromatic dispersion is used to refer to the dependence of group delay, xcfx84, on wavelength, xcex.
Linear (first order) dispersion, D, is the measure of the rate of change of group delay, xcfx84, with wavelength xcex. (D=dxcfx84/dxcex). Linear dispersion is typically measured in picoseconds per nanometer (ps/nm). In the case of a transmission medium, for instance, an optical fibre waveguide, whose waveguiding properties are uniform along its length, the linear dispersion exhibited by the medium is proportional to its length and so, for such a medium, it is convenient to define its linear dispersion per unit length, also known as its linear dispersion power. This is typically measured in picoseconds per nanometer per kilometer (ps/nm/km).
The value of the linear dispersion of a transmission path is generally itself a function of wavelength, and so there is a quadratic (second order) dispersion term, Q, also known as dispersion slope, which is a measure of the rate of change of linear dispersion with wavelength. (Q=dD/dxcex=d2xcfx84/dxcex2). This is typically measured in picoseconds per nanometer squared (ps/nm2). In some, but not all instances, the effects of quadratic dispersion in NDS and DC fibre (non dispersion shifted fibre, and dispersion compensating fibre) are small enough not to assume significance. There are also higher dispersion terms, whose effects generally assume even less significance.
In a digital transmission system the presence of dispersion leads to pulse broadening, and hence to a curtailment of system reach before some form of pulse regeneration becomes necessary. The problem presented by dispersion increases rapidly with increasing bit rate. This is because, on the one hand, increasing the bit rate produces increased spectral broadening of the pulses, and hence increased dispersion mediated pulse broadening; while on the other hand, increasing the bit rate also produces a reduction in the time interval between consecutive bits. In a WDM digital transmission system, it is not practical to minimise the problems of dispersion by choosing to employ a transmission medium exhibiting near-zero first order dispersive power because low first order dispersive power is associated with aggravated non-linear (e.g. four-wave mixing) distortion.
A known solution to this problem is to employ xe2x80x9cmanaged dispersionxe2x80x9d in which near-zero aggregate linear dispersion over a particular transmission path is achieved by the use of alternating sections respectively exhibiting positive linear dispersion and negative linear dispersion, for instance by the use of non-dispersion-shifted (NDS) and dispersion compensating (DC) optical fibre waveguide.
However, broad band dispersion compensating modules based on dispersion compensating fibre cannot provide sufficient accuracy to compensate all channels in a WDM system simultaneously.
Another solution has been to use dispersion compensation devices based on spectrally distributed reflection of optical signals from waveguides incorporating chirped Bragg gratings (i.e. gratings in which the effective pitch neff. xcex9 varies along the grating""s length, where neff is the effective refractive index and xcex9 is the physical pitch). Light of a particular wavelength xcex will, in effect, be reflected from a point along the grating at which the condition:
xcex=2neffxc2x7xcex9xe2x80x83xe2x80x83(1)
is satisfied. Thus, the chirped Bragg grating exhibits/provides chromatic dispersion because signal components at different wavelengths will be reflected, effectively, at different positions along the grating""s length, and so will have been delayed by different amounts of time when they reemerge from the waveguide after reflection.
The use of both linearly chirped and quadratically chirped gratings for dispersion compensation purposes are known. Also known are adjustable dispersion compensation devices in which the effective pitch of a Bragg reflection grating is adjusted by applying uniform or non-uniform strain (to alter physical pitch) or by applying a thermal gradient (to alter effective refractive index). For a grating with uniform physical pitch, controlling the magnitude of the thermal gradient controls the magnitude of the resulting chirp, and thus there is provided a form of adjustable amplitude linear dispersion compensation device. Such a device is for instance described by B J Eggleton et al. in, xe2x80x9cDispersion compensation in 20 Gbit/s dynamic nonlinear lightwave systems using electrically tunable chirped fibre gratingxe2x80x9d, Electronics Letters Vol. 35, No. 10, pp 832-3.
However, it is difficult to manufacture very long fibre Bragg gratings ( greater than 1 m) required for broadband compensation (i.e. for dispersion compensation across the entire band occupied by signals in a WDM system.
For optimum performance in such a system it is desirable to actively adjust the dispersion of each channel independently, to minimise transmission degradation. The known methods based on straining or temperature tuning of silica-based waveguides are in use but have a limited range. Also, it has been necessary to use large numbers of slightly different designs to support high channel count WDM systems.
Etalon-based devices have also been used for dispersion compensation purposes, but have insufficient dispersion and slope uniformity within the information bandwidth to be applicable in high spectral density, high bit rate WDM systems.
Embodiments of the present invention therefore aim to provide dispersion compensation apparatus, and corresponding methods, which overcome, at least partially, one or more of the above-mentioned problems/disadvantages associated with the prior art.
A further object of embodiments, not least for reasons of minimising inventory, for operating in a WDM system, is to provide a Bragg grating dispersion compensator that is capable of providing dispersion compensation for any individual one of the channels of that WDM system.
According to a first aspect of the present invention there is provided dispersion compensation apparatus comprising a waveguide comprising a sampled Bragg grating extending along a length of the waveguide, the sampled Bragg grating exhibiting a comb-link reflectance versus wavelength spectrum comprising a plurality of teeth; and adjustment apparatus arranged to adjust an effective refractive index of the waveguide along at least a portion of said length. By making this adjustment, the positions of the teeth in the spectrum may, for example, be shifted to bring one of the teeth into register with a bandwidth of a signal input to the waveguide, and/or the dispersion exhibited by the grating may be adjusted.
Sampled Bragg gratings are known. In non-sampled gratings, the effective refractive index along the waveguide structure is modulated in some way (e.g. in a periodic fashion) to produce the grating xe2x80x9celementsxe2x80x9d.
In a sampled grating, the depth of the refractive index modulation is itself modulated in some fashion.
In preferred embodiments, the sampled gratings comprise a sequence of groups of grating elements, connected by sections of waveguide in which the effective refractive index is substantially unmodulated (i.e. they contain no grating elements). However, this is not essential, and a comb-like response may be exhibited by a sampled grating where the modulation depth is modulated in some other fashion (e.g. it just varies sinusoidally along the waveguide""s length, never reaching zero).
Thus, in this specification, the term xe2x80x9csampled Bragg gratingxe2x80x9d is intended to encompass any Bragg grating arranged to provide a comb-like reflectance characteristic (i.e. repeated pass/reflection bands).
The simplest way to imagine a sampled grating is as follows:
Consider a conventional grating.
Make an identical grating with the coupling coefficient M times higher.
Leave the first N periods intact, erase the next (Mxe2x88x921)xc3x97N, and repeat.
We now have a sampled grating with the same average coupling strength as before.
Thus, the sampled grating consists of a sequence of groups of grating elements (i.e. grating samples) spaced apart along the length of the waveguide in which the grating is formed.
The sampled grating described above will give approximately the same peak reflectance as the original unsampled grating, with the pass-band repeating at a frequency which depends on the sample period.
Thus, whereas the unsampled grating exhibits a single reflectance band, the sampled grating exhibits a comb-like reflectance characteristic, with the spacing of the xe2x80x9cteethxe2x80x9d of the comb being determined by the optical path length between the samples. This optical path length depends on the physical separation of the samples and the effective refractive index of the portions of the waveguide connecting the samples. This comb-like characteristic results from the fact that each short grating sample acts as a broad-band reflector. Reflected signal components at a wavelength xcex from adjacent samples will constructively interfere if
xcex=2kxc2x7neffxc2x7Bxe2x80x83xe2x80x83(2)
where k is an integer, neff is the effective refractive index of the waveguide connecting the samples, and B is the sample period.
Each xe2x80x9ctoothxe2x80x9d will have substantially the same shape and width as the single reflectance band provided by the corresponding unsampled grating. Furthermore, when a signal whose bandwidth lies within a particular tooth is reflected by the sampled grating, the signal has imparted to it substantially the same dispersion as would be imparted to a corresponding signal on reflection by the corresponding unsampled grating.
Thus, sampling a grating results in a repetition of its reflectance (including dispersion) characteristics at a plurality of positions in the wavelength spectrum. The reflection characteristics corresponding to a particular tooth in the reflection spectrum are determined by the sampled grating as a whole, and the positions of the teeth are determined by the sampling period and refractive index along the grating.
The dispersion compensation apparatus according to the first aspect of the present invention provides the advantage that, by employing a sampled grating having a comb-link reflectance spectrum, then provided that a signal bandwidth lies within the comb it is only necessary to adjust the refractive index along the waveguide by a small amount in order to bring one of the teeth of the comb into register with the signal bandwidth to reflect it. Thus, tuning may be achieved quickly.
Furthermore, the same dispersion compensation apparatus can be used to compensate for dispersion in any signal having a bandwidth lying within the comb characteristics, so the same apparatus can be used to compensate for dispersion in all channels of a WDM system. Rather than requiring a large number of dispersion compensators of different designs, a corresponding number of nominally identical designs in accordance with the first aspect of the present invention can be used.
A further advantage is that, by adjusting the effective refractive index along the grating""s length, the dispersion provided to a signal on reflection may be adjusted/controlled.
Preferably the adjustment apparatus is arranged to control the average value of effective refractive index over the length. This may be achieved by making a uniform change to the value of the refractive index all along the length, or by simply altering its values at specific points along that length.
Preferably, the adjustment apparatus is also arranged to set a desired distribution of effective refractive index along this length. In this way, the dispersion exhibited by the device to incident optical signals can be controlled and adjusted.
Preferably, the grating is pre-chirped, but alternatively a chirp may be introduced by suitable adjustment of the refractive index profile along the length.
It will be apparent that by arranging for suitable distributions in refractive index, adjustable amounts of linear dispersion, quadratic dispersion (dispersion slope) and even higher orders of dispersion may be compensated.
In certain preferred embodiments, the control of effective refractive index along the length of the waveguide is achieved by controlling the temperature profile of a cladding material over, under, or surrounding a waveguide core.
In alternative preferred embodiments, the refractive index control is achieved by using a waveguide which has a core and a plurality of slots interrupting the core, each slot being filled with a material having a high temperature coefficient of refractive index or a refractive index which is a function of applied electric field. By controlling the temperature or electric field applied to the material in each slot a desired average effective refractive index or distribution along the length of the waveguide can be achieved.
Preferably, the slots are arranged between each and every pair of adjacent grating samples along the waveguide, such that by adjusting the effective refractive index of the material in the slots, the optical path lengths between the grating samples, and hence the reflectance properties of the grating, can be controlled.
A further aspect of the present invention provides dispersion compensation apparatus comprising: a waveguide comprising a sampled Bragg grating extending along a length of the waveguide, the sampled Bragg grating exhibiting a comb-like reflectance versus wavelength spectrum comprising a plurality of teeth; and adjustment apparatus for controlling an effective refractive index profile of the waveguide along said length.
Such apparatus provides the advantage that the positions of the teeth and/or the dispersion exhibited by the device can be adjusted so as to compensate for dispersion in any signal having a bandwidth lying within the comb spectral characteristics.
A further aspect of the present invention provides a waveguide comprising: a core formed from a first material; a sampled Bragg grating comprising a sequence of grating samples spaced apart along said core; and a plurality of waveguide sections formed from a second material, each section at least partially interrupting the core and being arranged in the optical path between a respective pair of adjacent grating samples, the second material having a higher thermal coefficient of refractive index than the first, whereby the optical path lengths between the samples, and hence the reflectance characteristics of the grating, may be adjusted by controlling the temperature of the sections.
In yet a further aspect, the second material is a material whose refractive index is dependent on applied electric field.
According to a further aspect of the present invention there is provided a method of compensating for dispersion in an optical signal having a bandwidth, the method comprising the steps of: inputting the signal to a waveguide comprising a chirped sampled Bragg grating extending along a length of the waveguide, the chirped sampled Bragg grating exhibiting a comb-like reflectance versus wavelength spectrum comprising a plurality of teeth; shifting the position of said teeth in the reflectance spectrum by adjusting the effective refractive index of the waveguide along at least a portion of said length, such that one of said teeth spans said bandwidth; and reflecting the optical signal from the chirped sampled Bragg grating.
Preferably, the shifting is achieved by adjusting the effective refractive index at a plurality of positions along the length, preferably by the same amount so as to increase or decrease the average effective refractive index along the length. Preferably each position lies between a respective pair of adjacent samples of the sampled Bragg grating.
Preferably, the shifting is achieved by altering the temperature or the electric field applied to portions of the waveguide between each pair of adjacent grating samples.
Preferably the method further comprises the step of altering the dispersion exhibited by the waveguide by adjusting the effective refractive index of the waveguide along at least part of its length so as to set a desired variation in effective refractive index over that length.
This effective refractive index xe2x80x9cprofilexe2x80x9d may be achieved by applying a continuous variation in effective refractive index over the length, or by controlling refractive index at a plurality of distinct positions along the length. These separate positions are preferably located between adjacent grating samples.
The desired distribution of effective refractive index along the length may be achieved by controlling the temperature of or the electric field applied to sections of the waveguide distributed along its length.
According to a further aspect of the present invention there is provided a method of compensating for dispersion in an optical signal having a bandwidth, the method comprising the steps of: inputting the signal to a waveguide comprising a sampled Bragg grating extending along a length of the waveguide, the sampled Bragg grating exhibiting a comb-like reflectance versus wavelength spectrum comprising a plurality of teeth; chirping the sampled Bragg grating by adjusting the effective refractive index of the waveguide at a plurality of position along said length to set a variation in effective refractive index along said length; shifting the positions of said teeth in the reflectance spectrum by further adjusting the effective refractive index at said plurality of positions, such that one of said teeth spans said bandwidth; and reflecting the optical signal from the chirped, sampled Bragg grating.
Thus, in this aspect, the sampled Bragg grating may initially be unchirped, with all of the chirp for dispersion compensation purposes being provided by the applied effective refractive index variation along the length of the waveguide.
According to a further aspect of the present invention there is provided a method of compensating for dispersion in an optical signal being a bandwidth, the method comprising the steps of: inputting the signal to a waveguide comprising a chirped sampled Bragg grating extending along a length of the waveguide, the chirped sampled Bragg grating exhibiting a comb-like reflectance versus wavelength spectrum comprising a plurality of teeth; controlling the profile of the waveguide""s effective refractive index along said length to adjust the position of the teeth in the spectrum to bring one of the teeth into register with the signal bandwidth to reflect the signal, and to adjust the dispersion imported to the signal by said reflection.
Further aspects of the present invention provide adjustable filters, line amplifiers, and receivers for WDM optical communications systems incorporating waveguides with sampled Bragg gratings in which effective refractive index can be controlled over the grating""s length.
Further aspects are defined in the claims.
Other features and advantages of the invention will be readily apparent from the description of the preferred embodiments of the invention, from the drawings and from the claims.